COMP 110: Introduction to Programming

COMP 110:Introduction to Programming

Tyler JohnsonMar 30, 2009

MWF 11:00AM-12:15PMSitterson 014

COMP 110: Spring 20092

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Program 3 has been graded


Questions?


Today in COMP 110

Review from last time

Arrays in Classes & Methods

Intro to Sorting

Programming Demo


Arrays

Review from last time


Arrays

A special kind of object in Java used to store a collection of data

What if you wanted to store 80 basketball scores?

Instead of declaring 80 integer variables, declare a single array!


Array Details

Syntax for creating an array:Base_Type[] Array_Name = new Base_Type[Length];

Example:int[] pressure = new int[100]; //create 100 variables of type int

that can //be referred to collectively

Alternatively:int[] pressure; //declare an integer array called pressurepressure = new int[100]; //allocate memory for the array to hold

100 ints


Arrays

The array itself is referred to by a name “scores” or “vector” (in our examples)

0 1 2 3 4

68 73 57 102 94

Indices

the array scoresscores[3]


Array Length

An array is a special kind of object

It has one public instance variable: length

length is equal to the length of the arrayPet[] pets = new Pet[20];int sizeOfArray = pets.length; //sizeOfArray will have the value

20

You cannot change the value of length (it is final)


For Loops and Arrays

For loops are often perfectly suited to processing arrays

Why?Because we know the number of iterations (array.length)

int[] pressure = new int[100];for(int index = 0; index < pressure.length; index++) scores[index] = 0;


Be Careful with Indices

Indices MUST be in bounds

double[] entries = new double[5];entries[5] = 3.7;

Your code WILL compile with an out-of-bounds index

But it will result in a run-time error (crash)

//RUN-TIME ERROR! Index out of bounds


Arrays in Classes & Methods

Section 7.2 in text


Arrays as Instance Variables

Arrays can be used as instance variables in classespublic class Pressure {

private double[] pressure;}

public class Course {

private Student[] enrolledStudents;}


Arrays as Instance Variables

public class Pressure {

private double[] pressure;

//ask user for the number of pressure values and read them inpublic void getData() {

System.out.println("Hi. How many pressure values would you like to enter?"); Scanner keyboard = new Scanner(System.in); int num = keyboard.nextInt(); pressure = new double[num]; System.out.println("Ok. Please enter " + num + " integers"); for(int i = 0; i < pressure.length; i++) { pressure[i] = keyboard.nextDouble(); }}

}

An array instance variable

Create storage for the array

Write values into the array


Arrays of Objects

Creating an array of objects does not initialize the individual objectsStudent[] students = new Student[35];

students[0].getGPA(); //run-time error, students[0] holds no object

Each object in the array must be instantiatedstudents[0] = new Student();students[1] = new Student();…students[34] = new Student();

Do this in a loop


Student[] students = new Student[3];for(int i = 0; i < students.length; i++) {

students[i] = new Student();}

1045 2584 2836

Arrays of Objects

major

class

GPA

major

class

GPA

major

class

GPA

? ? ?students

Remember: The value of an object is a memory

address!


Indexed Variables as Arguments

Indexed Variablescores[0], pressure[4], etc.

The same as using a regular variablepublic void printNum(int num) { System.out.println(num);}

public void doStuff() {

printNum(7); //prints 7 out to screen

int[] scores = { 15, 37, 95 };

for(int i = 0; i < scores.length; i++) { printNum(scores[i]); //prints the ith score out to screen }}


Array Assignment & Equality

Arrays are objectsThe value of an array is a memory address

Using == to compare arrays two arrays a & b returns whether they point to the same memory address

int[] a = {4, 5, 6};int[] b = {4, 5, 6};

//a == b is false!//a.equals(b) is false as well!


Comparing Arrays

To determine whether two arrays hold the same data, compare the two arrays element by element

Lab 7 equals method


What is the Output?

int a = 7;int b = a; b = 8;

System.out.println("A: " + a);System.out.println("B: " + b);

A: 7

B: 8

Output

//a is not changed by this


What is the Output?

int[] a = {4, 5, 6};int[] b = a;b[0] = 3;

System.out.println("a: {" + a[0] + ", " + a[1] + ", " + a[2] + "}");

System.out.println("b: {" + b[0] + ", " + b[1] + ", " + b[2] + "}");

a: {3, 5, 6}

b: {3, 5, 6}

Output

//b holds same memory address as a//we’re changing both b & a!

This is like giving the array two names (a & b)


Array Assignment

int[] a = {4, 5, 6};int[] b = a;b[0] = 3;

4 5 6a

b

3


Copying Arrays

int[] a = {4, 5, 6};int[] b = new int[a.length]; //create a new array b

//copy a’s entries into bfor(int i = 0; i < b.length; b++) {

b[i] = a[i];}


What is the Output?

public void changeNumber(int num) {num = 7;

}

public static void main(String[] args) {int a = 9;changeNumber(a);System.out.println("a = " + a);

int num = 9;changeNumber(num);System.out.println("num = " + num);

}

a = 9

num = 9

Output


What is the Output?

public void changeNumber(int num) {num = 7;

}

public static void main(String[] args) {int[] a = {4, 5, 6};

changeNumber(a[0]);System.out.println("a: {" + a[0] + ", " + a[1] + ", " + a[2] + "}");

}

a: {4, 5, 6}

Output


What is the Output?

public void changeArray(int[] array) {array[0] = 7;

}


changeArray(a);System.out.println("a: {" + a[0] + ", " + a[1] + ", " + a[2] + "}");

}

a: {7, 5, 6}

Output


What is the Output?

public void changeArray(int[] array) {array = new int[3];array[0] = 7;array[1] = 5;array[2] = 6;

}



}

a: {4, 5, 6}

Output


Graphical Example

public void changeArray(int[] array) {array = new int[3];array[0] = 7;array[1] = 5;array[2] = 6;

}



}

4 5 6a

array 7 5 6

a: {4, 5, 6}

Output


Programming Demo

Sorting (Selection Sort)

Section 7.4 in text


Introduction to Sorting

Given an array of numbers, sort the numbers into ascending order

Input array:

Sorted array:

4 7 3 9 6 2 8

2 3 4 6 7 8 9


Selection Sort

4 7 3 9 6 2 8

2 7 3 9 6 4 8

2 3 7 9 6 4 8

…


Pseudocode

for i = 0 to array.length - 1Find the index s of the smallest element starting at index iSwap elements i & s in the array

2 3 7 9 6 4 8

i s


Decomposition

Methodsint indexOfSmallest(int[] array, int startInd)• Return the index of the smallest element in array

starting at startInd

void swap(int[] array, int a, int b)• Swap the elements at indices a & b in array


Programming Demo

Programming


Wednesday

Multi-Dimensional Arrays

Documents

COMP 110: Introduction to Programming